The present invention relates to electrophotography and more particularly, to hard magnetic core particles for use in electrophotography that can be produced by a low temperature process.
In electrophotography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
Typically, carrier particles used in a rotating core development system contain a core made of a hard magnetic ferrite material such as SrFe12O19 having a single-phase, hexagonal crystal structure. Methods of preparing magnetic ferrite materials are described, for example, in U.S. Pat. Nos. 3,716,630; 4,623,603; 5,332,645, and 4,042,518, the teachings of which are incorporated herein by reference in their entirety; European Patent Application No. 0 086 445; “Spray Drying” by K. Masters published by Leonard Hill Books London, pages 314-317 and “Ferromagnetic Materials”, Volume 3 edited by E. P. Wohlfarth and published by North-Holland Publishing Company, Amsterdam, N.Y., Oxford, pages 502-509, the teachings of which are also incorporated herein by reference. In particular, commercially-prepared SrFe12O19 core particles are typically prepared by a method in which Fe2O3 and SrCO3 powders are combined with a binder and the mixture is spray dried to form green beads, which are subsequently fired, typically at a temperature of about 1300° C. In this method, an amount of SrCO3 is used in excess of the amount required to provide a 6/1 ratio of FeO3 to SrO (SrCO3 is converted into SrO during the firing.). The excess amount of SrO helps to densify the green beads.
A disadvantage of the commercial method of making SrFe12O19 using excess SrO is that at the normal firing temperature, the grain growth is rapid and uncontrolled, resulting in variable coercivity of the material. Further, the densification process provides hard sagger ingots that must be deagglomerated to recover the original bead particle size distribution. The deagglomeration process can result in a loss of 15-20% of the material through a classification step to remove the fines that are produced by deagglomeration. Further, the retained particle distribution contains fractured and irregular beads.
A further disadvantage of the commercial method of making SrFe12O19 is that the formulation that is used requires high temperatures in the range of 1300° C. Reduced firing temperatures do not yield sufficiently densified cores to achieve optimum density.
A further disadvantage of the commercial method of making SrFe12O19 is that excess SrO leads to the formation of surface salts that can impact triboelectric properties. In particular, excess SrO transforms to Sr(OH)2, and then to Sr(OH)28H2O, and eventually to SrCO3. These compounds affect the charge of the core particles and influence the charge to mass ratio (Q/m) of solution-coated and dry-coated carriers that are prepared from the bare core particles. The variability in the amount of these compounds that are formed on individual core particles affects the uniformity of the core particles.
Accordingly, there is a need for a method of forming hard magnetic core particles, wherein a lower temperature can be used.
Further, there is a need for a method of forming hard magnetic core particles, wherein the extent of deagglomeration required after firing is reduced, leading to a reduction in product loss and a reduction in fractured or irregular particles.
Further, there is a need for a method of forming hard magnetic core particles, wherein the formation of surface salts resulting from the transformation of excess SrO is avoided.
Further, there is a need for a method of forming hard magnetic core particles wherein the particles have a constant coercivity and Q/m even when process and formulation variables are varied.